This invention relates to infrared detectors comprising at least one detector element sandwiched between a substrate and an optical element, and relates particularly but not exclusively to such infrared detectors comprising an array of detector elements of cadmium mercury telluride with a corresponding array of immersion lenses as the optical element.
As described in published United Kingdom patent application GB-A-2 132 757 (and the corresponding United States patent specification U.S. Pat. No. 4,629,892), it is known to form an infrared detector comprising a detector element which has a body of infrared-sensitive semiconductor material (for example, cadmium mercury telluride) and which is sandwiched between a substrate and an optical element in the form of a lens, the lens being secured over the element body by an adhesive film. In this manner, optical immersion of the detector element is obtained so as to increase its sensitivity. The whole contents of GB-A-2 132 757 and U.S. Pat. No. 4,629,892 are hereby incorporated herein as reference material.
In the prior art to which GB-A-2 132 757 refers, the adhesive film is present between the detector element and the lens so as to bond the detector element to the lens. However its presence was considered in GB-A-2 132 757 to give rise to difficulties in infrared transmission through the adhesive, in differential thermal expansion leading to structural failure and in adversely affecting a passivated surface of the element body. The invention disclosed in GB-A-2 132 757 provides three small radially-disposed lens contact pads around a single detector element and avoids the provision of the adhesive film between the detector element and the lens. Thus, in accordance with the invention in GB-A-2 132 757, the adhesive film is provided only on the three radially-disposed contact pads which are formed of the same material as the element body.
Immersion lenses and other optical concentrators have also been formed as an array in an optical plate as described in United Kingdom patent specification GB-A-1 525 562, the whole contents of which are hereby incorporated herein as reference material. Similar arrays have also been formed in an optical plate for infrared regions of the spectrum, for example the wavebands 3 to 5 .mu.m (micrometers) and 8 to 14 .mu.m. Pending United Kingdom patent application (GB) 8522539 of Sep. 11, 1985 (published as GB-A-2 241 605 on Sep. 4, 1991) describes an array of infrared detector elements (for example of cadmium mercury telluride) which is bonded to such a lens-array plate by an adhesive film. Such an arrangement is illustrated in FIGS. 1 and 2 of the accompanying drawings.
In the array arrangement previously proposed by the applicant in GB-A-2 241 605 and shown in FIGS. 1 and 2, the adhesive film 11 extends between the element bodies 1 and the lens plate 10 and fills the space between the element bodies 1. The detector elements are infrared photodiodes having a vertical p-n junction 3 dividing the body into a p type region 2 and an n type region 4. The element bodies 1 are mounted (for example by a further adhesive film, not shown in the drawings) on a substrate 20 carrying electrical connections 22 and 24 to respective electrodes 5 and 6 of the regions 2 and 4.
Upon bonding the lens plate 10 to the element bodies 1, the applicant has noticed two unexpected changes in the characteristics of the photodiodes, namely a decline in the cut-off wavelength (for example by almost 1 .mu.m at a wavelength of about 9 .mu.m) and an increase in the junction resistance (for example by about 30% for the vertical junction diodes illustrated). FIG. 3 is a graph illustrating the shift in the cut-off wavelength .lambda. in .mu.m. The reciprocal of .lambda. is plotted as the ordinate against the operating temperature T of the detector element in degrees Kelvin as the abscissa. The line A is for the detector element material without the lens plate 10 bonded thereto, whereas line B is for the bonded element structure of FIGS. 1 and 2 with a silicon lens array 10 and an epoxy adhesive film 11.
The applicant has found that this shift in cut-off wavelength (and the change in junction resistance) can be significantly reduced by adopting a detector structure in accordance with the present invention.